Ascending tracts

Question 1

Types of modalities

Answer 1

Proprioception
2 point discrimination
Vibration
Temperature
Pain
Pressure
Stretch

Question 2

What determines quality of somatic sensation

Answer 2

Sub type of receptor

How the receptor is activated

Question 3

Classes of somatosensory receptors with example

Answer 3

Phasic - mechanoreceptors (frequency of impulses decrease with prolonged stimulation)
Tonic - nociceptors (frequency of impulses constant for given stimulation over time)

Question 4

Describe general pathway of somatosensory afferent neurones

Answer 4

Primary afferent neurone: cell body in DRG, synapses in spinal cord (crosses from PNS to CNS)

Secondary afferent neurone: crosses midline, synapses in thalamus

Tertiary afferent neurone: cell body in thalamus, synapses in primary sensory cortex

Question 5

What is the receptive field of a somatosensory neurone

Answer 5

Area of skin supplied by 1 primary afferent neurone

Question 6

Relationship between receptive field and acuity

Answer 6

Inversely proportional

Question 7

Describe lateral inhibition

Answer 7

Stimulated primary afferent neurones excite inhibitory interneurones to enhance contrast to help localise stimulation when >1 neurone stimulated.
The peripheral neurones also inhibit the central neurones but NOT BY AS MUCH

Question 8

What is a spinal tract

Answer 8

1 way white matter connection between 2 areas of grey matter

Question 9

What are the ascending tracts

Answer 9

Dorsal columns

Spinothalamic

Question 10

What is the medial lemniscus pathway

Answer 10

Secondary afferent neurone in the dorsal column tract

Part of dorsal column between the medulla and thalamus

Question 11

What are the 2 parts of the dorsal column tract

Answer 11

Fasciculus gracilis medially

Fasciculus cuneatus laterally

Question 12

Which modalities do the ascending tracts control

Answer 12

Dorsal column - light touch, vibration, proprioception, 2 point discrimination

Spinothalamic - crude touch, pain, temperature

Question 13

Describe the pathway of dorsal column neurones

Answer 13

Primary: travels up spinal cord more medially if from lower dermatome and synapses in medulla

Secondary: crosses midline in medulla and synapses in thalamus more medially from lower dermatome

Tertiary: synapses in postcentral gyrus more medially if from lower dermatone

Question 14

Describe the pathway of spinothalamic neurones

Answer 14

Primary: enters spinal cord and synapses immediately (dorsal horn?)

Secondary: crosses midline of spinal cord, travels up spinal cord more medially if from upper dermatome, synapses in thalamus more medially if from upper dermatome

Tertiary: synapses in postcentral gyrus more medially if from lower dermatone

Question 15

Differences between main pain fibres

Answer 15

A: carry impulses from mechanoreceptors, small receptive field, myelinated, produce sharp pain (localised)

C: polymodal, large receptive field, unmyelinated, produce dull pain (poorly localised), higher threshold

Question 16

What type of neurones do pain fibres stimulate

Answer 16

Secondary afferent neurones of the spinothalamic tract in the spinal cord

Question 17

Describe descending control of pain

Answer 17

With high pain/anticipation of pain the periaqueductal gray in midbrain activates the nucleus raphe magnus which activates inhibitory enkephalinergic interneurones that synapse on secondary afferent neurones of spinothalamic tract

Question 18

How does rubbing a painful area alleviate pain

Answer 18

Activates mechanoreceptors in skin which stimulates A fibres.
A fibres activate branching inhibitory enkephalinergic interneurones that synapse on secondary afferent neurones of spinothalamic tract

Question 19

Test for proprioception loss

Answer 19

Romberg’s test - ask patient to walk on spot with eyes closed (removes visual input to balance)
Loss of balance either due to loss of proprioception or vestibular system pathology